Why does the uterus have thick muscular walls?

The uterus has thick walls because a fertilized egg would need a thick lining in order to implant and begin to grow and develop. The uterus is sterile and has no bacteria present, however large quantities of debris and bacteria is brought into the uterus with the sperm. Therefore, the walls must be strong and muscular because uterine contraction has to occur to expel fluid, dead sperm and bacteria out from the uterus and through the open cervix.